Outdoor electrical equipment cabinets are often used to protect sensitive electronic equipment from tampering, vandalism and adverse environmental conditions. Examples of the latter include high and low temperatures, rain and other forms of precipitation, and various types of particulates, such as dust, pollen and soot. One common use for outdoor cabinets is to house telephone equipment, such as the channel banks that are used to carry out analog-to-digital and digital-to-analog conversion between subscriber lines and telephone company lines. Advances in the state of the art have allowed channel banks and other types of telephone equipment to become smaller and to operate at higher power densities. In addition, some newer types of equipment (such as fiber optic equipment) are increasingly sensitive to dirt and other kinds of contamination. In order to address these needs, newer cabinet designs have had to dissipate heat more efficiently, and have also had to provide a greater degree of isolation from the outdoor environment.
An accepted practice in the manufacture of outdoor telephone equipment cabinets is to seal the cabinet from the outdoor environment in order to prevent contamination of the cabinet interior, and to dissipate internally generated heat using air-to-air heat exchangers. These devices exchange heat between the air in the cabinet and the ambient air outside the cabinet, without allowing the interior and ambient air to mix. In this way, a substantially closed or sealed environment can be maintained within the cabinet, while still allowing for adequate heat dissipation. Typically, the heat exchanger is located in the main portion of the cabinet, in proximity to the electronics requiring cooling. However, the presence of the heat exchanger in the cabinet can interfere with the cables used to interconnect the various electronic components, making the routing of these cables somewhat more difficult. Also, since the heat exchanger occupies a significant amount of space, the cabinet must usually be made larger to accommodate it. Thus, for example, if the heat exchanger is located near the roof of the cabinet, as is sometimes the case, the height of the cabinet must usually be increased. Similarly, if the heat exchanger is located at the side or end of the cabinet, the length or width of the cabinet must usually be increased. Unfortunately, any increase in the size of the cabinet is undesirable, since these cabinets are often placed in residential areas and must be as small and as low in height as possible to avoid aesthetic objections.
There has been at least one attempt to mount a heat exchanger onto the door of an electrical equipment cabinet, an arrangement which, at least in theory, makes more efficient use of the space available within the cabinet. In U.S. Pat. No. 4,535,386, to Frey, Jr. et al, a heat exchanger in the form of a corrugated sheet is secured to the inner surface of the door of a sealed enclosure containing heat-generating electronic devices. The corrugations form passageways through which ambient air rises on the outside of the enclosure, thereby cooling the side of the heat exchanger facing the interior of the enclosure. This causes the air inside the enclosure to be cooled and to sink downwardly along the interior surface of the corrugated sheet, thereby creating natural air turbulence within the enclosure.
Although the heat exchanger of U.S. Pat. No. 4,535,386 is relatively simple and compact in construction, the reliance on purely convective air flows inherently limits its cooling capacity. In cases where the equipment within the cabinet is tightly packed and has a high power density, or in cases where the configuration of the cabinet is such that the area of the door is relatively small relative to the volume of the cabinet, these convective air flows may not provide adequate cooling. The addition of fans to force air through the heat exchanger would present additional difficulties, since the mounting of fans on the door would require that electrical power be supplied to the door through a flexible cable or the like. This would increase the complexity of the door structure considerably, and would also render the door more susceptible to damage or failure due to external impacts, shocks from repeated opening and closing, and other factors.